The present invention is directed to a current limit circuit. In particular, the current limit circuit of the present invention is an integrated circuit that can be used with a metal output resistor.
Current limit circuits are employed to protect devices and loads from excessive damaging currents. They fall into two main classifications--direct current limit schemes and indirect or approximate current limit schemes. The present invention is directed to an approximate current limit circuit. A direct current limit circuit senses the entire available current. An approximate current limit circuit samples only a portion of the total current. The sampling element since it is not ideal, tends to introduce first order errors which in many cases are not compensated for.
FIG. 1 depicts an approximate current limit circuit of the prior art. If the operation of the components of this conventional circuit are considered to be ideal and assuming the Vbe of the output transistor Q0 equals the Vbe of the sense transistor Q1, the current in the output transistor is limited to: ##EQU1## Thus, the current limit amount is sensitive to the reference voltage Vref, the current sense resistor R.sub.0, the supply voltage Vs and a resistor ratio Rsns/R.sub.2. However, additional thermal errors are introduced into the equation when the base-emitter voltage of power output transistor Q0 is not equal to the base-emitter voltage on the sense transistor Q1. Additional errors may be introduced by mismatching of the sense resistor Rsns and the output resistor R0 due to manufacturing tolerance. Also, where the output resistor R0 is a different type of resistor than the sense resistor, for example, a metal resistor as opposed to a diffusion resistor, the resistor may react differently to temperature changes causing further errors.
Temperature will have no effect if there are ideal emitter areas and current ratio as well as thermal matching. However, since the power output transistor Q0 and the sensing transistor Q1 are physically apart from one another, the instantaneous junction temperature across the power structure is significantly higher than that on the sensing transistor due to the larger total power generated at the output transistor. The higher temperature rise lowers the base-emitter voltage on the output power transistor. This in turn exponentially increases the output current causing further reduction of the base-emitter voltage initiating a positive thermal feedback loop. This in some cases may lead to thermal runaway resulting in "hot spots" and eventual structural failure. In a thermal runaway situation, the current limit circuit loses control of the output current. However, the thermal conductivity of the silicon allows the heat to propagate from the output transistor to the sensing transistor so as to develop a negative thermal feedback by which the current limit circuit can eventually regain control.
In conventional circuits such as that disclosed in FIG. 1, some thermal gradient will always exist and is obviously a function of the lateral distance between the circuit elements and the package type. Interestingly enough, a device utilizing a power package with a heat sink will experience a larger thermal gradient per lateral dimension than without a heat sink. A heat sink introduces a lower vertical thermal resistance causing an increase in the lateral thermal gradient
The conventional approximate current control scheme of FIG. 1 controls the emitter currents. Since the output transistor Q0 and the sensing transistor Q1 are subjected to different voltages at their collectors, they have different betas, thus, errors in the collector currents may be generated. This may also be seen in that while the ratio of the base-emitter currents of the output transistor and sensing transistor is fixed, the base-emitter current in the output transistor is supplied from both the collector and the base thereby permitting variations in the collector currents. This variation in the output current is referred to as the early voltage effect. Such an early voltage effect would be found in a conventional current limit circuit such as that disclosed in U.S. Pat. No. 3,845,405 (Leidich).